Bryolichen Communities, Forest Maturity Indicators I: Lichens

Every type of forest ecosystem contains precious botanical treasures of great value to those who study them and to the general public: exuberant plants with spectacular flowers, majestic trees with delicious fruits, epiphytes like orchids with their astonishing adaptation to pollenization by insects. However, briophyte and lichen communities, though notable, often pass unnoticed even though they play an important role in the ecosystem. One can find them at soil level (terricolas), on rocks (saxicolas) and at all heights of trees (epiphytes) – inferior and superior trunks and branches – that is to say, that wherever one looks they are there, always there, filling every small space of the forest, producing, providing and protecting.

Lichens are complex organisms comprised of two parts, the mycobiont (fungus) and the photobiont (algae or cyanobacteria), which allows them to perform photosynthesis and to collect atmospheric nitrogen. The fungus and alga have a mutualistic relationship where each benefits from the other, but they maintain a fragile equilibrium. The photobiont (algae) is the least specialized of the two and lives at the limit of its niche; however, together they can live in environments that they could not live in seperately. Lichens are present at all latitudes, from the equator to the poles and at all altitudes, from the beach to the summits of mountains. Returning to the algae or cyanobacteria, because they live at the extreme end of their biological limits, they are the most sensitive to changes in environmental conditions, regardless of what type of change. Contamination, light exposure, humidity, temperature, etc. can severely affect the photobiont and hence the lichen.

Lichen grow very slowly because they require a certain temperature and humidity that is only present for a few hours a day (Ederra Induráin, 1996). Because of their sensitivity to change and narrow biological niche, their condition, presence or absence, are good indicators of long term environmental stability and health (ibid).

In a natural ecosystem with little or no human impacts, observing where and how lichen are growing in the soil, on rocks and trees can show the maturity of the ecosystem. Lichen that grow on rocks (saxicolas), especially those that are totally adhered to the rock, grow much slower than those that grow on leaves (foliaceous). One should look for when lichen start to appear in Analog Forest restoration projects. If the same lichen communities as found in a mature forest are present in an Analog Forest, then the restoration is well advanced.

In tropical and subtropical forests with abundant vegetation, lichen communities are usually found on all trunks and branches and are almost always associated with mosses, with which they occasionally line the entire trunk. In general, these organisms are not considered parasites nor do they cause any structural or physiological damage; although, there are some studies that demonstrate the contrary with some species, for example, Evernia spp. They only use the substrate for support and do not take nutrients from it. They do, however, help to retain moisture, provide habitat for insects and make atmospheric nitrogen available to the ecosystem.

Lichens are excellent biological indicators of the maturity of the forest because of its perfect integration in the ecosystem, and furthermore, like the rest of the living organisms, it contributes to the natural equilibrium of the system.